As production method of a toner for electrostatic development, a kneading and grinding method has been widely used. A toner for electrostatic development obtained by that method tends to has a wide particle size distribution and has a lot of coarse powders and fine powders. Therefore, it is pointed out that the deterioration of image quality and a carrier pollution by the toner easily occur. Furthermore, for producing a toner having a small particle diameter and a narrow particle size distribution by the kneading and grinding method, much energy may be required in some cases.
As a method for producing an aqueous dispersed system of a resin microparticle, a method using a twin screw extruder has been disclosed (refer to Patent Document 1). That is a method comprising dissolving a polyester resin in an organic solvent to have a viscosity capable of emulsification and adding water thereto for the phase inversion emulsification. Since that method employs an organic solvent, a process for removing such a solvent is troublesome and there is also a problem of economical efficiency. Further, it is difficult to completely remove an organic solvent from the aqueous dispersed system. Therefore, there are problems of environmental contamination, safety, odor and the like caused by the organic solvent.
Meanwhile, there has also been proposed a method comprising melting a raw material for a toner comprising a polyester resin, adding water to the melted product for the phase inversion emulsification to form a resin microparticle, and agglomerating and fusing the formed resin microparticle to obtain a toner (refer to Patent Document 2). In the Patent Document 2, a microparticle having a particle diameter of not less than 2.4 μm before fusion has only been disclosed. In that method, since a particle diameter of the microparticle before fusion is big, there has been a problem that it is not possible to obtain a resin microparticle having physical properties that the present inventors have targeted.    Patent Document 1: JP1998-139884A    Patent Document 2: JP2002-351140A